Development and characterization of a fiber-optic monitoring system for the key environment variables of the spent nuclear fuel pool at a nuclear power plant

This study develops and characterizes a fiber-optic monitoring system for the key environment variables of the spent nuclear fuel pool (SNFP) at a nuclear power plant. The three key environmental variables indicating the SNFP status directly are its water temperature, water level, and radiation level. First, this study develops and characterizes the individual fiber-optic sensors for measuring the three key environmental variables and then assembles them into an integrated monitoring system. The individual fiber-optic sensors commonly use optical fibers to transmit the signals delivered from their sensing probes despite their different characteristics. For the fiber-optic temperature sensor (FOTS), two types of FOTS are developed: contact and non-contact types, which are distinguished by whether their sensing probes are in direct contact with water. The contact-type FOTS uses a copper metal cap as its sensing probe, and the non-contact-type FOTS uses an infrared optical fiber, whose peripheral surface is coated with an anti-fog solution as its sensing probe. The fiber-optic water level sensor (FOWS) consists of optical fibers with their ends connected to the sensing probes fabricated with a NaCl solution and stainless steel. The FOWS measures the water level using the Fresnel reflection phenomenon, i.e., reflection of a portion of incident light at a discrete interface between two media having different refractive indices. The FOWS identifies the water level by measuring the amount of light reflected at the interface between the sensing probe and its outside medium, which varies according to whether the sensing probe is in contact with water. The fiber-optic radiation sensor (FORS) measures the gamma radiation in the SNFP. The sensing probe of FORS is a cylindrical-shaped LYSO:Ce scintillator, whose peripheral is wrapped with aluminum foil as the reflector. After characterizing the three individual sensors developed in this study, they are assembled and tested at a model water pool, 500 mm × 500 mm × 500 mm in size. The performance test results shows that individual sensors can measure the changes in each environmental variable in real-time.